1. Technical Field
The present disclosure relates generally to an assembly for determining the level of solid particles in a vessel, including storage vessels such as chip bins and silos, as well as processing vessels such as chemical digesters and impregnation vessels. The present disclosure relates more particularly to a method and system for detecting the level of wood chips or other biomass used in the manufacture of paper pulp or other products from lignocellulosic biomass.
2. Related Art
The present disclosure describes a solids level indicator. Although the solids level indicator may be used in processes or industries involving the movement of solids or fluids or both solids and fluids through a vessel, subsequent exemplary may refer to chip bins, impregnation vessels, and continuous digesters used in the pulp and paper industry.
Mill operators use mechanical solids level indicators in holding vessels such as silos, storage bins, pipes, and treatment vessels such as chemical digesters and impregnation vessels to estimate the height of the pile of solids within the vessel. For example, in the pulp and paper industry, mill operators may use one or more continuous chemical digesters to produce pulp. Continuous digesters tend to be large, vertically oriented reactor vessels that tower over a pulp mill. These digesters use caustic liquors to break down pretreated lignocellulosic material into paper pulp and byproducts. Although methods vary, operators generally feed solids such as softened wood chips or other lignocellulosic material into the top of a continuous digester. Generally, the continuous digester is either a vapor phase continuous digester or a hydraulic phase continuous digester.
Inside a vapor phase continuous digester, the chips collect in a conical pile. The conical pile gradually sinks under its own weight into a mire of caustic liquor and proto-pulp. The caustic liquor denatures lignin, a protein in the wood chips that naturally holds the cellulosic fibers together. As the protein itself dissolves into the liquor, the wood chip's cellulosic fibers disintegrate into viscous proto-pulp. The liquor and proto-pulp ooze slowly down the length of the vertical vessel until the liquor and proto-pulp reach an outlet port at the bottom of the continuous digester. At the bottom, operators pump the proto-pulp to downstream equipment for further processing and cleaning. In hydraulic phase continuous digesters, the liquor level generally exceeds the chip level. Mechanical solids indicators allow operators to calculate the top level of the chips and thereby monitor pulp production and control the rate at which new chips enter the vessel.
Typical mechanical level indicators generally have a shaft disposed in a shaft housing that extends through a pipe in the vessel wall. The shaft housing has an exterior end bolted to the vessel wall. In this manner, the shaft housing generally resists torsional movement. The shaft may be hollow and is disposed within the shaft housing. The hollow portion of the shaft may house a sensor as shown in U.S. Pat. No. 3,971,254, the entirety of which is incorporated herein by reference. The sensor is typically a strain gauge. The shaft housing's interior end typically has a paddle extending generally perpendicularly to the length of the shaft housing and the paddle is typically oriented transversely to the direction that the solids flow. That is, a single paddle generally extends from the side of the shaft housing and crosses the path of the usually downward-moving solids. As the solids move through a vessel, the solids exert a force on the paddle. The paddle, being connected to the side of the shaft housing, transforms the downward force of the solid particles into a torsional force. The shaft within the shaft housing, being in torsional communication with the paddle, communicates the torsional force to a strain gauge or other sensors, located within the shaft.
Although the shaft may or may not rotate visibly, the sensors measure the torsional force (e.g. the twisting force) and shear stress on the shaft. The degree to which the solids particles impart torsional force and create shear stress on the shaft housing and shaft can be used to ascertain the mass of solids above the paddle. The mass, together with the known density of the solids can be used to calculate the position of the top level of the solids pile. Furthermore, in holding vessels and treatment vessels, solid level indicators may be disposed at several elevations along the height of the vessel.
Solids level indicators may also have shear pins, which extend through the shaft housing into the shaft. These shear pins generally prevent the shaft from rotating within the shaft housing. Shear pins generally withstand less torsional force than the paddle or shaft itself. If the torsional stress on the paddle and shaft becomes excessive, the shear pins typically break before the paddle or shaft breaks. When a shear pin breaks, the paddle rotates downwardly to a neutral vertical position. That is, the paddle no longer crosses the path of the solid particles. While this protects the paddle, a paddle in the neutral position can no longer detect the downward force of the chips and therefore can no longer measure the chip level. While generally successful, this design has several drawbacks.
If the solids level indicator is used in a digester, impregnation vessel, or other treatment vessel that operates under high temperature and pressure, the high temperature may damage the electronic sensors within the hollow shaft and the temperature may cause the solids level indicator to expand unevenly. Uneven expansion can disrupt readings. Additionally, manufacturers generally laminate multiple sensors inside of the hollow portion of the shaft. If the mass of solids is sufficiently large, the solid particles may bend and distort the hollow shaft and shaft housing. Small distortions in the shaft straightness may dislodge or damage the sensors, thereby rendering the solids level indicator non-functional. Shaft distortion may further distort the paddle. Laminating these sensors or servicing these sensors requires precision and the efforts of a skilled worker in a temperature-controlled environment.
Extracting a solids level indicator from an active treatment vessel would expose an operator to fluctuating, unsafe process conditions. For these reasons, these sensors generally cannot be installed at the mill site on an active treatment vessel such as a continuous digester or impregnation vessel. As a result, the mill operator generally shuts down the mill before extracting and replacing damaged solids level indicators. If the mill operator does not have spare solids level indicators available, the operator ships the broken solids level indicators to the manufacturer to be rebuilt. Replacing the sensors, and pretreating the solids level indicators to protect the solid level indicators from process conditions (such as caustic liquor, high temperatures and corrosive steam) can add weeks to a mill shutdown, which results in loss of production and reduced work hours for mill employees.
To avoid some of these problems, manufactures developed solids level indicators with magnetic sensors fixed to the exterior ends of the shaft housing and shaft such as the one depicted in U.S. Pat. No. 4,964,301, the entirety of which is incorporated herein by reference. However, to obtain accurate readings, the entire solids level indicator had to be precisely designed to the operating conditions of a particular continuous digester. For example, the length and thickness of the shaft housing and shaft would vary depending upon whether the digester was a vapor phase digester or a hydraulic phase digester. Additionally, the paddle area would vary depending not only upon the type of digester but also upon the location of the solids level indicator within the digester. For example, if one imagines a vapor phase digester and a hydraulic phase digester having substantially the same operating capacity, then the area of hydraulic phase digester paddles are generally greater than the area of vapor phase digesters paddles. The chips in a hydraulic phase digester are suspended in liquid and therefore generally exert less force on the paddles; the paddles generally have a larger surface area to compensate. However, mill operators tend to use hydraulic phase digester plates in the middle and bottom process zones of vapor phase digesters because they notice that the liquor level can rise into these process zones. Unfortunately, the liquor level in vapor phase digesters can fluctuate and return below the solids level indicator in the middle and bottom process zones. When the liquor falls below these solids level indicators, the liquor no longer offsets the weight of the chips, and the chips can bear down on the large-area paddles with excess force. The excess weight can surpass the loads for which the paddles were designed, which breaks the shear pins. As a result, the paddle on the solids level indicator rotates 90 degrees to a position in which the paddle no longer crosses the path of the solids. In this manner, the solids level indicator may be saved but rendered non-functional.
Inactive solids level indicators reduce the accuracy of top level measurements. To the extent operators extrapolate other process conditions from the top level data, top level inaccuracies can reduce the accuracy of other process measurements. This may in turn increase the risk that improper processes regulation will create safety hazards, or loss of production due to premature maintenance shutdown.
Accordingly, there is a long felt need to have a solids level indicator that can provide accurate level measurements within a vessel while overcoming the limitations of the prior art.